and on the Function of the Antennce in Crustacea. 37 



laying bare the cepbalic ganglion, we find that the two anterior 

 branches of nerves pass to the eyes, the central being the motor 

 oculi, the outer the ophthalmic, and lead direct to the organs 

 of vision. 



In all the higher tribes, except the aberrant family of Dia- 

 stijlidce (Say) [Cuma of M. Milne-Edwards), the eyes are borne on 

 moveable pedicles. In the Brachyura the ring which bears these 

 pedicles is free, but unlike Squilla, instead of being seen distinctly 

 in advance of the animal, it is enclosed within and covered by 

 the anterior portion of the carapace ; I say that it is covered by, 

 and not absorbed into, the structure of the integument which 

 forms the anterior portion of the external skeleton of these 

 animals. 



If we throw off the carapace we shall find that the calcareous 

 representation (PI. I. fig. 1 a) of the ophthalmic ring occupies a 

 position between and connecting the two eyes, lodged in a fossa 

 (PI. I. fig. 2 w) formed by the interspace between the dorsal and 

 ventral arches of the second ring, the superior antennal, which 

 arches approximate in the Brachyura so closely, that, as in the 

 genus Cancer, they meet in front of and enclose the ophthalmic 

 ring, leaving the point of union visible only by a distinct suture 

 (PL I. fig. 3 a). Thus the ophthalmic ring is covered by and 

 not fused with the rest of the testaceous skeleton ; — it therefore 

 takes no part in the development of the carapace of the Decapod 

 Crustacea. 



The superior antennse succeed the eyes, and with the excep- 

 tion of the genus Squilla, the ring which supports them is always 

 fused with the succeeding, the inferior antennal. These two 

 form a closely associated part in the anterior structure of the 

 animal, and together build up the whole of that portion of the 

 carapace which is in advance of the cervical sutvire, and which, 

 I think, I shall be able to show, forms almost the whole of the 

 carapace in the Brachyura, — half of the same in Macroura, — and 

 that it lessens in importance as the animal descends in the scale 

 of nervous centralization. 



If we turn our attention to the lower forms, we find that in the 

 Cuma and other allied genera of the Diastylida, the eye (for the 

 two coalesce so as to form but one) is developed nearly in the 

 centre of the carapace ; but this appearance is only the result of 

 the great development of the lateral angles of the carapace, 

 which meet in front and form what appears like a rostrum ; 

 they never unite, but ai'c distinctly separated through the centre 

 of the so-called rostrum, as well as on either side of that portion 

 which supports the antennal rings, which occupies a small island 

 as it were in the centre of the carapace (PI. I. figs. 4, 5 & 6). 



The fact which dissection has enabled us distinctly to make 



